1. Field of the Invention
This invention relates to an apparatus and method for internal combustion engine control capable of suppressing the emission of nitrogen oxide from an internal combustion engine to the atmosphere.
2. Description of the Related Art
There is known a method for improving the fuel economy or other performance of an internal combustion engine by controlling the air-fuel ratio to a target value (for example, 22) leaner than the theoretical air-fuel ratio (14.7) to thereby carry out lean-combustion in the engine when the engine is driven in a predetermined driving state. However, if a three-way catalytic converter is used for the engine to which the aforesaid method is applied, nitrogen oxide (NOx) cannot be sufficiently purified during the lean-combustion since the three-way catalytic converter does not operate at its full potential in the lean air-fuel ratio range. In this respect, attempts have been made to reduce the emission of NOx even in the lean-combustion driving by using a so-called NOx catalyst, which absorbs NOx discharged from the engine in an oxygen enriched state (oxidizing atmosphere) and deoxidizes the adsorbed NOx in a hydrocarbon (HC) excessive state (reducing atmosphere).
However, there is a limit to the amount of NOx which can be absorbed by the NOx catalyst. If the engine is continuously driven in the lean-combustion mode, the catalyst will be saturated with NOx. In this case, most part of NOx gas discharged from the engine is emitted to the atmosphere. To obviate this, before or when the NOx catalyst is saturated with adsorbed NOx, a shift is made to the rich mixture control, which controls the air-fuel ratio to a theoretical ratio or its near value to thereby start the theoretical ratio operation or rich-combustion operation of the engine. The resultant exhaust gas containing plenty of unburnt gases creates a reducing atmosphere for deoxidization of NOx around the catalyst.
Regarding the timing at which the lean-combustion operation is switched to the theoretical ratio operation or rich-combustion operation, a method is known from Japanese Patent Application KOKAI Publication No. H5-133260 in which the elapse time from the start of lean air-fuel ratio control is measured, and the changeover to rich air-fuel control is forcibly carried out when a predetermined time has elapsed. With this method, the lean air-fuel ratio control is started again upon completion of deoxidization of NOx adsorbed by the catalyst during the rich air-fuel ratio control. In this way, the lean-combustion and rich-combustion are alternately effected to reduce the emission of NOx.
However, according to the control disclosed in the aforementioned Publication, if the predetermined time has elapsed from the start of the lean air-fuel ratio control, it is always determined that the amount of adsorbed NOx has reached the saturated amount. In this instance, the lean-combustion mode is forcibly changed to the rich-combustion mode. Thus, the improvement of fuel economy by the lean-combustion cannot be sufficiently achieved, and the fuel economy will be lowered accordingly. Moreover, at the time of changing the air-fuel ratio, the engine torque varies to give a bad influence on the engine operation. In a vehicle using the engine as the primary drive, an engine torque variation can cause a shock similar to acceleration shock, and hence the drive feeling will be deteriorated if the engine torque variation frequently occurs while the vehicle is driven at a constant speed. Further, the emission of HC increases when the air-fuel ratio is enriched. Therefore, from the viewpoint of improving the drive feeling and reducing the emission of HC, it is not preferable to forcibly and frequently change the air-fuel ratio.
Disclosed in International Patent Publication No. W093/08383 is a technique for deoxidizing NOx adsorbed by the exhaust purifying device during the lean-combustion driving of an engine. In this technique, the adsorption amount of NOx by the exhaust purifying catalyst is estimated, and the driving mode is changed to the rich-combustion mode when it is determined based on the result of estimation that the adsorption amount of NOx has reached the saturation amount.
However, according to this technique, in which whether or not the catalyst is saturated with NOx is determined based on the estimated adsorption amount of NOx, if the NOx adsorbing abilities of catalysts mounted to individual engines are different from one another, it is sometimes impossible to correctly determine that the catalyst has reached the saturated state.